If you get a call from a US phone number and don't recognize who it is, look at the number and remember what digits follow after country code “1”. The next three digits are the area code, which means approximate location of the caller. To find out which state you can call from use the tool “Lookup by area code”, which will speed up the search. By three digits it is impossible to know exactly who is calling your phone and from where, but in the event of a call back, you will at least know long distance call cost.